The principles of nuclear magnetism have been applied to the measurement of blood flow utilizing the proton of the water fraction of blood. Specifically, the nuclear magnetic resonance phenomenon (NMR) is used to enhance the detection of these protons once they have been polarized by a strong, steady, magnetic field. Two different systems have been used: 1) A two magnet system using a high-level field polarizer and a low-field homogeneous detector, 2) A one magnet system using a high-level, homogeneous field which serves as polarizer and detector. Both systems can employ an erasing field at a specified point between polarizer and detector to determine the proportion of blood passing through the erasing field. A specific aim is to provide the necessary instrumentation for measuring regional intracranial blood flow to facilitate studies of intracranial atherosclerosis and stroke.